The two pressed on, though, continuing their search for objects beyond Neptune. They had a new base of operations: the Institute for Astronomy at the University of Hawaii, with its access to the world-class cluster of telescopes atop Mauna Kea. Other groups began looking as well. Between 1989 and 1991, several teams of astronomers at various other institutions tried and failed to find distant solar system objects. As did Jewitt and Luu. But CCD technology was improving by leaps and bounds. Larger and more sensitive chips became available, and Jewitt and Luu took full advantage of them.
On the night of August 30, 1992, they hit pay dirt. Using a 2.24 m telescope on Mauna Kea, they spotted an object that seemed to move as they blinked two successive images of the same part of the sky. Additional observations in the following weeks verified their discovery: a tiny planetesimal[6] in an orbit beyond Neptune, and it was officially announced in September. The Minor Planet Center in Cambridge, MA, which keeps track of these things, gave the object the preliminary designation 1992 QB1. We now know that 1992 QB1 is a fairly small body, probably about 200 to 250 km in diameter assuming an albedo or surface reflectivity of 0.04 (similar to a cometary nucleus). Its orbit has a semimajor axis[7] of 43.7 AU[8], with perihelion at 40.9 AU (about 1.6 billion kilometers beyond Neptune) and aphelion at about 46.6 AU. It has an inclination of 2.2 degrees and a minuscule eccentricity of 0.065. 1992 QB1 appears to be a rather typical classical Kuiper Belt object.
* * * *
[Footnote 6: Small bodies formed in the early solar system by accretion of dust and ice.]
[Footnote 7: The average distance from the object to its primary, such as a planet or comet from the Sun.]
[Footnote 8: Astronomical unit, the mean distance from the Sun to the Earth, about 149.6 million km.]
* * * *
The largest known objects in the Kuiper Belt
Name/Designation: Pluto
Diameter (km): 2,320
Semi-major axis (AU): 39.4
Discovered: 1930
* * * *
Name/Designation: 90482 Orcus
Diameter (km): ~1,600
Semi-major axis (AU): 45
Discovered: 2004
* * * *
Name/Designation: Charon
Diameter (km): 1,270
Semi-major axis (AU): 39.4
Discovered: 1978
* * * *
Name/Designation: 90377 Sedna
Diameter (km): 1,000-1,500
Semi-major axis (AU): 532
Discovered: 2004
* * * *
Name/Designation: 50000 Quaoar
Diameter (km): 1,000-1,400
Semi-major axis (AU): 43.25
Discovered: 2002
* * * *
Name/Designation: 55636 2002 TX300
Diameter (km): ~965
Semi-major axis (AU): 43.19
Discovered: 2002
* * * *
Name/Designation: 55637 2002 UX25
Diameter (km): ~910
Semi-major axis (AU): 42.71
Discovered: 2002
* * * *
Name/Designation: 28978 Ixion
Diameter (km): 890-1,220
Semi-major axis (AU): 39.39
Discovered: 2001
* * * *
Name/Designation: 20000 Varuna
Diameter (km): 860-1,260
Semi-major axis (AU): 43.23
Discovered: 2000
* * * *
Name/Designation: 55565 2002 AW197
Diameter (km): 770-1,010
Semi-major axis (AU): 47.52
Discovered: 2002
* * * *
The Borderland of Sol
Almost a thousand planetesimals belonging to the Kuiper Belt have been discovered since 1992, and astronomers have steadily gathered basic information about them. The data acquired so far are giving planetary scientists new insights into the formation and evolution of the solar system's outermost regions. This is the area that science fiction writer Larry Niven once called “The Borderland of Sol.”
How was the Kuiper Belt formed? Alan Stern is serious when he calls the Kuiper Belt the solar system's attic. For many of us, the attic of our house is a convenient storage space. It's where we toss all the stuff that we're not using and don't have a place for in the living room, bedroom, or even the closets. So up it goes, and out of sight.
The same appears to be true of the Kuiper Belt. The solar system formed 4.7 billion years ago from a disk of dust, ice and gas. Dust and ice particles stuck together, soon growing large enough for their gravitation to begin pulling in still more material. Planetesimals closer to the infant Sun were mostly rock, and soon merged into the four inner terrestrial planets. Much further out, planetesimals composed of both rock and ice grew. Several became huge—the infant Jupiter, Saturn, Uranus, and Neptune. In the region now occupied by Uranus and Neptune, planetesimals would have formed mostly from dust and ices of water, methane, and other simple organic molecules. These primordial objects would have soon settled into slightly elliptical orbits on the plane of the solar system.
Within a few million years, Jupiter had become massive enough for its gravitational field to exert a powerful influence on the evolving solar system. Computer simulations show that Jupiter's gravity would violently disrupt the orbits of planetesimals within its region of influence. Some would be thrown farther inward; others would be flung entirely out of the solar system. And still others would be tossed into orbits taking them dozens and even hundreds of AU from the Sun.
More important for the Kuiper Belt, though, was the formation of Neptune. Other computer simulations indicate that Uranus and Neptune may have formed closer to the Sun than they are today. As their gravitational fields threw planetesimals inward towards Jupiter, they slowly spiraled outward (an example of Newton's Third Law). As it did so, Neptune's gravitational field in particular affected the orbits of many planetesimals in the regions beyond its original location. Some became trapped in orbits affected by mean-motion resonance with Neptune; others were tossed even further outward. And some remained mostly where they were, but in orbits with greater eccentricity or inclination that before.
* * * *
Orbit of Quaoar, a classical KBO.
Illustration courtesy of Chad Trujillo, Gemini Observatory
* * * *
This is a fairly simplified picture of the Kuiper Belt's formation—a confusing and chaotic time in the solar system's early history. There are still many puzzles that await resolution. For example: Why does the Kuiper Belt have a fairly sharp boundary at about 50 AU? Is it a real edge, or merely a dip or “trough"? What creates the Kuiper Belt's dynamic structure?
What does the Kuiper Belt look like? Astronomers generally agree that the Kuiper Belt has three main structural components: the classical Kuiper Belt, a scattered disk, and an extended scattered disk.
The classical Kuiper Belt stretches from Neptune's orbit (about 30 AU) to a fairly sharp boundary at about 50 AU from the Sun. This appears to be real; it corresponds closely to the 2:1 mean-motion resonance point with Neptune, the strongest gravitational resonance at that distance. “This might be because the radial migration of Neptune shoved pre-existing disk objects out that far and no more,” says Jewitt. “The edge could also be caused by tidal truncation from a passing star or conceivably from a distant, unknown ‘planet'” or planet-sized KBO.
The vast majority of KBOs found so far are in this region. Orbits in the classical Kuiper Belt are stable over the age of the solar system, mainly because their perihelia are far enough from Neptune that its gravity can't scatter or disrupt them. About 10 to 25 percent are called Plutinos; like Pluto, they're in 3:2 mean-motion resonance with Neptune. Other KBOs are in orbits with different resonance relationships with Neptune. “I consider them a sub-population of the classical belt,” says Brett Gladman of the University of British Columbia, the discoverer or co-discoverer of many KBOs. Jewitt considers them to be a separate component of the Kuiper
Belt. “Resonant objects are clearly a stand-out group on plots of semi-major axis versus eccentricity,” he says.
The objects in the classical Kuiper Belt have fairly circular orbits with semimajor axes from 35 to 56 AU, and form a region that looks like a doughnut. Some KBO orbits have a relatively high inclination or eccentricity. Within that doughnut is a region of orbits with low inclinations and low eccentricity. Gladman has referred to this latter region as the “dynamically cold component” of the classical Kuiper Belt, and the more inclined or eccentric orbits the “dynamically warm region.” ("Cold” and “warm” refer to the relative mean random energy of orbits in each population, as if the KBOs were particles in a gas.) For reasons still unknown, the KBOs in dynamically warm orbits are grayer or more neutral in color, while those in the dynamically cold orbits are reddish in color. The problem is, there shouldn't be any dynamically “warm” component in this region; the orbits should have retained their low eccentricity and inclination for the age of the solar system. So something “stirred up” the classical Kuiper Belt early in its history, jostling some objects into dynamically warm orbits.
“We still don't know for sure what causes the correlation between inclinations and colors,” says Chad Trujillo, a co-discoverer of several of the largest KBOs found so far. He thinks the two components may be two populations of KBOs superimposed on one another. “I think it's pretty easy to imagine that some inner solar system interlopers"—Gladman's dynamically warm group—"could have been superimposed onto the older, redder core population that formed beyond Neptune,” he says. Trujillo also makes a sharper distinction between “the classicals” and “the resonant” KBOs, which he says “almost all [have orbits] quite a bit more elliptical than the classicals.”
Other possible reasons for the dynamically warm component include those invoked to explain the sharp edge to the classical Kuiper Belt—
A planetesimal the size of Pluto or Mars passed very near or through the Kuiper Belt.
Another star passed within 50 to 200 AU of the outer parts of the Kuiper Belt during the early eons of the solar system.
As Neptune slowly migrated outward from its original location closer to the Sun, its mean-motion resonance regions disrupted the inclination of KBO orbits beyond Neptune.
The scattered disk is a region of KBOs with orbits that are highly eccentric, have large semi-major axes, and perihelia at around 35 to 40 AU, relatively near Neptune's orbit. Their aphelia are way out there: from several hundred to 3,000 AU distant.
The orbits of the scattered disk objects (SDOs) are stable on billion-year timescales, but their perihelia mean they do have a weak gravitational interaction with Neptune. In fact, that's probably why scattered belt objects are scattered. Repeated encounters with Neptune would send some of them deeper into space and pull others into orbits taking them closer to Neptune, Uranus, and the outer planets.
Finally, we should mention the Centaurs, defined as objects like 2060 Chiron with perihelia in the realm of the giant planets Jupiter and Saturn. Centaurs are not part of the scattered disk now. Says Gladman, “They have semi-major axes that are less than 30 AU, and spend most or all of their time within Neptune's orbit.” But they may have originally come from the scattered disk, tossed further inward by coming too close to Neptune.
* * * *
Orbit of Sedna, a KBO that may actually be the first discovered member of the Oort Cloud.
Illustration courtesy of Chad Trujillo, Gemini Observatory
* * * *
The extended scattered disk includes KBOs with large semi-major axes, large eccentricities, and with perihelia greater than 38 AU. These objects never come close enough to Neptune—"the big boy out there,” says Jewitt—for their orbits to be jostled or distorted by its gravitational field. There are only two KBOs currently known to be part of the extended scattered disk, 90377 Sedna and (possibly) 2000 CR105. Orbits in the extended scattered disk are stable over the age of the solar system. But that also means that their orbits could not have been created by mean-motion resonances with Neptune. Some other force or forces must have put these KBOs in these orbits.
“People have successfully made models using a close stellar passage to create such orbits,” says Trujillo, “so I would say it's a valid explanation. A Mars-sized body at around twice the distance to Neptune might also make such a body but I don't think anyone has modeled this case. A close stellar passage is the best explanation so far, but I think it's still much too early to say for sure.”
Beyond the extended scattered disk is the inner region of the Oort Cloud. First postulated by astronomer Jan Oort in 1950, the Oort Cloud is the source of long period comets, which have orbital periods greater than 200 years and enter the inner solar system from every part of the sky. The Oort Cloud is thought to consist of an inner flattened region that ranges from 1,000 to 5,000 AU, and an outer spherical region that stretches out to 50,000 or even 100,000 AU—a third of the way to the nearest star.
“The observational difference between the extended disk and the inner Oort cloud is a subtle one,” says Alan Stern. “It's more semantic than anything; they probably blend into one another. As we learn more about KBOs we can determine which population they belong to by their physical characteristics. We can't really distinguish yet from their orbits alone.”
What are KBOs made of? KBOs are so distant and small that determining the exact composition of their surfaces and interiors is quite difficult. Astronomers instead gather information on the surface colors of KBOs, and then make informed guesses about their composition. KBOs range in color from somewhat neutral (gray, that is, reflecting all wavelengths more or less equally) to very red. The reddish KBOs may be covered by large areas of tholins, polymers formed when ultraviolet light strikes simple organic compounds like ethane and methane. Or, they may have the kind of blackened crusts seen on some comet nuclei. The more neutral-colored objects may have more ices on their surface. Collisions with other objects dig craters in dark, crusty surfaces and throw up fresher icy material, including water, carbon monoxide, carbon dioxide, and methane ices. Some of the larger KBOs may even have active ice volcanoes like Triton. The basic belief among planetary scientists is that most KBOs are probably made of various kinds of ices mixed with dust and some organic material. In this respect KBOs appear to have much the same composition as comets—an argument consistent with the Kuiper Belt as the source of short period comets.
How big are they? There are at least a half-dozen known KBOs that may be a thousand kilometers or more in diameter. The largest are 90482 Orcus (up to 1,600 km in diameter), 90377 Sedna (1,000-1,500 km), and 50000 Quaoar (1,000-1,400 km). And there likely are a lot more big ones out there.
“There are probably a hundred thousand Kuiper Belt objects that are a hundred kilometers or more in diameter,” says Stern. “There are at least dozens of objects—and maybe as many as a thousand—that are half the size of Pluto or bigger.” That's as many as a thousand Charons, and Sednas, and Quaoars. But wait, says Stern; there's more. “I'm a hundred percent sure in my bones that there's a least one Pluto- or Mars-sized object in the Kuiper Belt. And I'd say there's a fifty percent chance that there's one Kuiper Belt object the size of Earth.”
Trujillo agrees with this assessment, but adds, “I bet that if there is a Mars-sized KBO, it is probably very distant. I would be very surprised if it were, say, within 50 AU and we didn't know about it already.”
And smaller ones? Estimates range into the billions.
* * * *
New Horizons Primary Instrument Uses
Name: Ralph
Instrument Type: Imager/Imaging spectrometer
Primary Uses: *Panchromatic photometric/geologic mapping (1 km resolution); *3-color and CH4 mapping (3 km resolution); *Composition mapping (7 km resolution); *Thermal mapping (2-20 km resolution)
* * * *
Name: Alice
Instrument Type: UV imaging spectrometer
Primary Uses: *Atmospheric compos
ition; *Upper atmosphere pressure & temperature profiles
* * * *
Name: REX
Instrument Type: Radio science, radiometry
Primary Uses: *Lower atmosphere pressure & temperature profiles; *Disk averaged brightness temperatures; *Masses, J2s
* * * *
Name: LORRI
Instrument Type: High-resolution imager
Primary Uses: *Pluto far-side mapping (35-40 km resolution); *High-resolution geology (50 m resolution; *Distant/early start to encounter imagery (5x farther than Ralph)
* * * *
Name: SWAP
Instrument Type: In situ plasma spectrometer
Primary Uses: *Atmospheric escape rate
* * * *
Name: PEPSSI
Instrument Type: In situ particle spectrometer
Primary Uses: *Pickup ion composition
* * * *
Name: SDC
Instrument Type: In situ dust counter
Primary Uses: *1st solar system dust density profile beyond 18 AU
* * * *
New Horizons
Pluto is the only planet not visited by a space probe from Earth. And no one even knew for sure that the Kuiper Belt existed until 13 years ago. Planetary scientists have wanted to send a spacecraft to Pluto since the 1970s. Now, after years of ups and downs, cancellations and resurrections, a spacecraft from Earth is about to journey to the borderland of Sol. Its primary target: Pluto and Charon. Its secondary target: one or more Kuiper Belt objects. The mission is named New Horizons Pluto-Kuiper Belt. At the time this issue of Analog went to press, it was ready to be launched during a 36-day window beginning January 11, 2006 from Cape Canaveral atop an Atlas V booster.
New Horizons has a mass at launch of about 465 kg, with on-board power provided by the same kind of plutonium-powered radioisotope thermal generators used by the Voyagers, Galileo, and Cassini-Huygens space probes. The spacecraft carries seven scientific instruments, including cameras, spectrometers and radio science experiments. One instrument worth special mention is the Student Dust Counter (SDC), the first instrument designed and run by college students to be carried on a planetary space probe. Alan Stern is the principal scientist for the New Horizons mission.
Analog SFF, March 2006 Page 8
